UnsafeCell doc typos and minor flow improvements

[shepmaster]

r? @QuietMisdreavus

[rust-highfive]

The job x86_64-gnu-llvm-3.9 of your PR failed on Travis (raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

[00:05:26] travis_fold:start:tidy
travis_time:start:tidy
tidy check
[00:05:26] tidy error: /checkout/src/libcore/cell.rs:1246: line longer than 100 chars
[00:05:28] some tidy checks failed
[00:05:28] 
[00:05:28] 
[00:05:28] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/tidy" "/checkout/src" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "--no-vendor" "--quiet"
[00:05:28] 
[00:05:28] 
[00:05:28] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/tools/tidy
[00:05:28] Build completed unsuccessfully in 0:02:04
[00:05:28] Build completed unsuccessfully in 0:02:04
[00:05:28] make: *** [tidy] Error 1
[00:05:28] Makefile:79: recipe for target 'tidy' failed

The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:18d2f0da
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

[shepmaster]

Sorry for the overly-large diff. I hit the 100 character limit so needed to reflow it. I went ahead and filled that paragraph to 90 characters to give a bit of space for the next person who needs to edit it.

[QuietMisdreavus]

@bors r+ rollup

[bors]

📌 Commit 52249e3 has been approved by QuietMisdreavus

[bors]

⌛ Testing commit 52249e3 with merge 21b5367...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: QuietMisdreavus
Pushing 21b5367 to master...